Demos always get recording, but never saved, so the disk space is safe.
But if a player called a command "saveDemo" then demos will be saved with the format presented in "cl_autoDemoFormat" cvar.
Each call of the command saves the current in-game timestamp, so the idea is to call the command on only the needed moments.
So if we called it at 0:07 then at 13:37 on ffa_ns_streets in 1.02 version then the demo will have the name "0007_1337_ffa_ns_streets.dm_15" by default.
Autorecording gets restarted each new round.
If a player forgot or had no chance to save the demo they are still able to find it in LastDemo folder or save it with "saveDemoLast" command.

And another feature: new DEMO recording icon similar to REC in cameras. It's placed in the left bottom corner.

Boothand wrote:Didn't quite understand by the quote. Is it always recording with cl_autoDemo 1, or do you need to press a record button? If only "saveDemo" starts a new demo, how do I do that without keeping it?

If you have enabled the cvar, then it will start to record at the beginning of each round, but the demos won't be saved unless you use the saveDemo command